1. Field of the Invention
The present invention relates to an image forming apparatus. In particular, the present invention relates to an image forming apparatus and an image forming method each used for performing dither processing on image data to form an image subjected to pseudo-halftone processing.
2. Description of the Related Art
An image forming apparatus adopting electrophotography, such as a laser printer, carries out printing through processes including exposure, development, transfer and fixation.
In an exposure step, first, an electrostatic latent image of an image is formed on a surface of a photosensitive drum by irradiation with an exposure beam changed in amount on the basis of image data to be printed out. Herein, a gray scale of the image, that is, a gradation can be represented by intensity modulation of the exposure beam or control of an exposure time.
In a development step, next, toner is attached to the photosensitive drum, so that the electrostatic latent image is developed as a toner image on the photosensitive drum. In a transfer step, the toner image is primary-transferred from the photosensitive drum to a transfer belt, and then is secondary-transferred from the transfer belt to a sheet of paper for printing. In a fixation step, heat is applied to the sheet of paper, so that the toner image is fixated on the sheet of paper.
A color printer that uses multicolor toner requires an exposure unit and a development unit for each color. In this color printer, toner images of the respective colors are superposed on a transfer member such as a transfer belt, and then are transferred collectively to a sheet of paper for printing. This color printer is known as a so-called tandem color printer.
The foregoing image forming apparatus adopting photography has the following problem. That is, since the toner images of the respective colors are superposed on the transfer member, respectively, misregistration of the colors degrades image quality.
Upon printing of a uniform halftone image, for example, if misregistration of colors occurs partially, a portion where the misregistration occurs is different in color from the remaining portion in development. In other words, there is a possibility that color shading may occur in a page.
Further, if misregistration of colors in one page is different in degree from that in another page, there is a possibility that color variation may occur between the pages.
The color variation occurs due to different degrees of color misregistration, so that pitch nonuniformity occurs disadvantageously. Accordingly, such a disadvantage must be prevented.
For example, Japanese Laid-Open Patent Publication No. 62-091076 proposes a technique of growing a line screen in a sub scanning direction in order to eliminate pitch nonuniformity as much as possible. Alternatively, Japanese Laid-Open Patent Publication No. 2001-341356 proposes a technique of setting a size of a matrix at a value other than an integral multiple of beams in order to eliminate pitch nonuniformity due to multibeam irradiation as much as possible.
However, each of the foregoing methods fails to suppress color variation due to misregistration of colors.